Optoelectronic recording tape or strip comprising photoconductive layer on thin, monocrystalline, flexible sapphire base

ABSTRACT

An optoelectronic signal recording and storage medium including a base layer, a conductive layer, a photoconductive layer and storage layer has a coherent crystal morphology throughout, even though the chemical and electrical properties of its layers are by choice dramatically different. The base layer is preferably made of monocrystalline sapphire grown in a manner as to allow the growth of the other layers directly on a surface of the base layer without the need to grind and polish that surface, thereby minimizing internal defects in the medium. The monocrystalline base layer also allows the acceptance of exeptionally uniformly distributed charges over wide areas of the medium, thereby enabling the medium to locally record and store minutely differing optoelectronic signals on a background of minimal noise, thus facilitating low light level electronic or optical recording and long term storage of signals and minimal energy readout of those stored signals. The medium base layer can be thin enough to be flexible and transparent and yet to have great strength to provide a firm foundation for the other medium layers. A method of making the medium is also disclosed.

BACKGROUND OF THE INVENTION

This invention relates to the recording of optical signals andparticularly to an improved optoelectronic recording medium and a methodof making that medium.

The recording of optical signals, particularly in large volume and at ahigh rate, is usually accomplished either by indirect means or by directmeans.

In the indirect mode of recording, the optical signal, e.g., a lightwavefront, is received by an array of CCD's or other sensors andconverted to electronic signals which, in turn, produce magnetic signalsto be recorded on a magnetizable medium such as a magnetic tape or disc.

The direct method of recording, on the other hand, involves a directinteraction between the light signal, after it has been properlyfocused, shaped and geometrically arranged, and a light sensitive mediumfor direct storage. In this latter method, the storage medium is usuallyphotographic film or a photoconductive material such as selenium or zincoxide dispersed in a dielectric binder. The former type of medium isused mostly in photographic cameras, while the photoconductive medium isincorporated into office copiers.

The advantages of indirect recording include the ease of reading andprocessing the converted signal. That signal, recorded magnetically inserial fashion, is readily compatible with electronic circuits that canmanipulate and process the recorded information. Another advantage ofthe indirect mode of recording is the ease with which the informationcan be erased either partially or totally. In other words, in anindirect recording system, the optical signals, after having beenreceived and converted to magnetic form, possess the ease of handlingwhich produce the flexibility inherent in magnetic read/write/erasesystems. The principle disadvantages of the indirect recording methodinclude signal distortion introduced during signal conversion, the needto switch to a serial information handling format, the relatively lowupper limit of the bandwidth of the captured data stream, the relativelypoor signal-to-noise ratio of the recording medium and the relativelylow packing density of the data stored on the medium, i.e., the largevolume of tape or space required to store the original data stream.

The principle advantage of direct optical recording is the ease withwhich the incoming optical signal stream can be routed to the recordingmedium. The raw information is captured in analog form and stored in aparallel manner so as to retain the geometric relationships of all ofthe resolution elements contained in the incoming optical wavefront.However, conventional photographic recording techniques have severaldisadvantages which seriously limit their applications. These includelow efficiency during processing in the conversion of the light signalto an ionic chemical signal on the film, the failure to achieve energyreciprocity at signal durations faster than the microsecond range, theneed to process the acquired optical signal chemically in order to fixit to the film and the difficulty in accomodating the acquired signal tomatch the needs of standard electronic data processing circuitry.

Direct recording using known optoelectronic or photoconductive mediadoes not involve chemical processing. In this respect, then, it ispreferable to photography, prompting industry to devote considerableresources to improve this mode of data recordation. The efforts in thisregard have led to the development of a variety of direct recordingoptoelectronic film and plate structures. The ones that show the mostpromise comprise a photoconductive light modulating section and adielectric storage section. By exposing the modulating section to alight image, an electrical charge can be impressed on the storagesection whose spacial distribution over the area of the storage sectionis an electrical analog of the original image.

In one medium of this type, described in U.S. Pat. No. 2,825,814(Walkup), the light modulating section and the storage section areseparate structures which are assembled in use. That is, the modulatingsection comprises a photoconductive layer with a transparent conductivebase and the storage section is a dielectric layer with a transparentconductive base. In use, the photoconductive and dielectric layers areplaced in contact and a high voltage is applied between the conductivebases of the two sections, while a light image is projected onto theassembly. After a brief period, the light is turned off and the twomembers are separated leaving the light image stored on the dielectriclayer as an electrical charge distribution. The image can then bedeveloped by applying toner to that section. This type of recordingmedium is disadvantaged in many respects. These include the requirementof a high charging voltage with its attendant danger, the necessity ofassembling and disassembling the modulating and storage sections and thedistortions in the image-representing electrical charge on thedielectric layer due to the air gap inevitably present between theassembled sections.

Another type of recording medium which does not involve such assemblyand dissassembly of the modulating and storage sections of the medium isdescribed in Electrostatic Imaging and Recording by E. C. Hutter et al.,Journal of the S.M.P.T.E., Vol. 69, January 1960, pp. 32-35. This mediumhas a transparent organic plastic base layer, such as polyester film,coated on one side with a layer of photoconductive material which is, inturn, coated with a thin layer of a dielectric material. To record animage on the medium, the dielectric layer is precharged by a coronadischarge directed to that layer. Then, the photoconductive layer isexposed to a light image, while an electric field is applied across thedielectric layer. The charge in the dielectric layer decays towards zerowith the decay being most rapid where the optical image is brightestand, therefore, the photoconducter resistance the lowest. After a timecorresponding to the greatest difference between the potentials in thelight and dark areas of the medium, the electric field is turned off andthe discharging process stops thereby leaving on the dielectric layer anelectrostatic charge image corresponding to the optical image incidenton the medium. The stored image may be developed by applying toner tothe medium or it may be read from the medium by scanning the dielectriclayer with a focused electron beam as is done in a vidicon tube toproduce a capacitively modulated electrical signal corresponding to thestored image. While this medium is a unitary structure, a voltage mustbe applied to the medium prior to exposure in order to precharge thedielectric storage section. This increases the cost and complexity ofthe associated recording apparatus. Also, the image-representing currentsignal produced by such scanning has relatively poor quality and lowsignal-to-noise ratio. Furthermore, that scanning process requires asource of high voltage making that medium impractical for use in aportable self-contained instrument such as a microscope or camera whichrelies on battery power. The medium has several other disadvantages aswell which seriously limit, if not prevent, its practical application.More particularly, it has poor light sensitivity comparable to theslowest silver halide films. Furthermore, it can store the acquired dataonly for a limited period of time, e.g., a few weeks, because of chargeleakage in the dielectric storage layer of the medium. Furthermore, thatmedium is not physically strong or rugged enough to be practical forlong-term information storage. U.S. Pat. No. 3,124,456 (Moore) shows asimilar structure that is similarly disadvantaged.

Another type of multi-layer electrostatic storage medium which does notrequire precharging of the medium is disclosed in U.S. Pat. Nos.4,155,640 and 4,242,433 to Kuehnle et al. This medium comprises atransparent plastic substrate or base which carries a layer ofphotoconductive material, there being a conductive layer between thephotoconductive layer and the base. Superimposed on the photoconductivelayer is a layer of dielectric material and on top of that is anotherconductive layer completing the sandwich. In operation, a low DC voltageis applied to the sandwich between the two conductive layers while themedium is exposed to a light image through the transparent base. Thelight image causes the photoconductive layer to modulate the flow ofcharge carriers so that an electrostatic image is impressed on thedielectric storage layer. Thereafter, the conductive layer adjacent thestorage layer is stripped off so that a charge distributioncorresponding to the original light image remains on the dielectriclayer. The stored image can be developed by toner or read by electronbeam scanning. While that medium is a unitary structure, it does requirethe removal of the electrode layer from the storage section followingexposure in order for the image-representing charge to remain on themedium. This strippable conductor necessitates the presence of aconductive fluid or a fusible bonding layer between the conductor andthe dielectric layer in order to obtain the necessary intimacy betweenthe electrode and the dielectric. This complicates the manufacture ofthe recording medium and, in the case of the fusible bonding layer, itrequires the presence in the associated camera or recorder of a hot shoeor similar device to melt the bonding layer to permit removal of theconductor. That medium also is characterized by the presence ofso-called dark currents in its photoconductive layer which result incharge leakage from the dielectric layer. This makes that mediumunacceptable for signal storage over an extended period of time.

Yet another recording medium disclosed in U.S. Pat. No. 3,880,514(Kuehnle) avoids the requirement of a removable conductor to store animage on the medium. However, this is done by eliminating the dielectriclayer from the medium. Accordingly, that film can only store an imagefor a short time due to charge leakage through its photoconductivelayer.

Additional problems affecting all of the prior electrographic recordingmedia of which we are aware, including the phototapes and filmsspecifically discussed above, stem from the fact that the materials inall of those multi-layer structures are selected primarily for theirohmic electrical properties and general commercial availability, withminimal consideration being given as to how the various layers should beintegrated into a total overall structure which would achieveunprecedented performance. In fact, the layers in the prior structuresare made without attention to the interrelationship and thecompatibility of those layers. As a result, there are definitemechanical boundaries between the adjacent layers of the media which area source of internal electrical noise and inconsistencies. Also, variouslayers may differ in their degrees of perfection giving rise to poorsensitivity, a high noise level in the stored image and premature lossof that image.

Most critically, the importance of the substrate or base material ininfluencing dramatically the overall operation of the recording mediumhas been totally overlooked in the prior media. That is, electrographictapes and films such as those described above, usually utilize for thebase a polyester or other organic plastic material. Made as a thin filmor tape, this material is quite strong and flexible; also, it isoptically clear, at least initially. However, it is subject toelongation and distortion making it difficult to achieve a good bond oradherence of the light modulating section of the medium to the base.This problem can be alleviated to some extent by including a specialbonding layer between the substrate and the medium's modulating sectionas discussed in U.S. Pat. No. 4,269,919 (Kuehnle). On the other hand,that solution creates additional interfaces and boundaries in the mediumwhich are undesirable, as noted above. It also increases the complexityof the medium and the cost of making it since the formation of eachlayer in the medium involves a separate sputtering or coating process.Still further, while the plastic substrates of the prior flexible tapesand films may have excellent optical clarity when the medium is new, assoon as the medium is placed into service, its substrate reacts to theincident energy at the ultraviolet end of the light spectrum by losingits optical clarity, making the medium less responsive to low lightenergy levels These plastic substrates are not particularly scratchresistant either, so that the substrate surfaces often have scratcheswhich impair the medium in the same way.

All of those prior media discussed above with plastic substrates orother components are disadvantaged also because such organic materialinvariably suffers outgassing when the medium is placed in a vacuum.Bearing in mind that information should ideally be retrieved from thesemedia by electron beam scanning in a vacuum, it becomes apparent thatsuch outgassing will interact with the electrons in the scanning beamand adversely effect, to the point of commercial impracticality, theimage-representing electrical signals produced by the scanning process.

To avoid problems caused by such plastics, it has been proposed to makethe medium substrate out of an inorganic material such as metal orglass. However, those materials are quite stiff, opaque or fragile. Evenif monocrystalline wafers of silicon or sapphire were used, such asthose available from the integrated circuit industry, one would facemajor problems. This is because in order to make such inorganicstructures thin enough to be of use for applicant's purposes in anoptoelectronic medium, they must be ground and polished to such anextent that there is an excessive amount of breakage. Furthermore, thosewafers that do survive the finishing process have surface defects andabrasions caused by such finishing that degrade the bond with, andinitiate defects in, any layer of material that is added to the surfaceof that structure. These internal defects, in turn, reduce the purityand performance of the resultant film to the point of making it uselessand impractical as a recording medium for an optoelectronic camera orrecorder.

In general, then, while the prior electrographic recording media andprocesses may work in principle, they are not satisfactory in practiceand have never found commercial use. It should be understood in thisconnection that a suitable recording medium, for applicant's purposes,must be able to be erased completely and also be used a multiplicity oftimes without any appreciable loss of its strength, flexibility, opticalsensitivity or its data storage capability. To applicant's knowledge,none of the known recording media, including those described in theabove-identified publications, possess these capabilities and,therefore, none are suitable for the detection and recording of lowenergy optical signals and for the required long-term storage ofequivalent electrical signals which are necessary to obtain theabove-stated advantages of both direct and indirect recording.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedoptoelectronic imaging and recording medium.

Another object of the invention is to provide a medium of this typewhich possesses superior data acquisition and storage capabilities.

Yet another object is to provide an optoelectronic recording mediumwhich obtains the benefits of both direct and indirect recording.

A further object of the invention is to provide a very sensitiverecording medium which is responsive to very low light energy levels orelectronic pulses.

Another object of the invention is to provide such a medium which has anexceptionally high signal-to-noise ratio.

It is a further objective to provide an optoelectronic imaging andstorage medium which has a very wide spectral response from shortwavelength signals below the detectability threshold of the human eye toregions in the long wavelength, infrared end of the spectrum, featuringa substrate which transmits ultraviolet, visable and far-infraredradiation frequencies.

Another object is to provide a medium such as this which does not loseits desirable mechanical, electrical and optical properties through useor over time.

Still another object of the invention is to provide such a medium whichcan acquire and store optical or electrical signals covering a widerange of intensities.

A further object of the invention is to provide an improvedoptoelectronic recording medium which can store the acquired data for along time.

Another object of the invention is to provide such a medium which can beerased and reused repeatedly without losing or altering the desirablecharacteristics described above.

Another object of the invention is to provide a recording medium of thistype which may be made very thin and flexible, yet extremely strong anddimensionally stable so that it can be rolled up repeatedly on a spoolwithout any material degradation of the medium.

A further object is to provide such a medium that does not require anyseparable electrodes or other parts.

Still another object of the invention is to provide an optoelectronicrecording medium for storing optical and electrical signals so that thestored data can be read out conveniently and efficiently by electronbeam scanning without destruction of the recorded information.

A further object of the invention is to provide such a medium whichpermits refreshment of the data stored by the medium.

Yet another object is to provide a method of making a medium having oneor more of the above characteristics.

Other objects will, in part, be obvious and will, in part, appearhereinafter. The invention accordingly comprises the sequence of stepsand the features of construction, combination of elements andarrangement of parts which will be exemplified in the following detaileddescription, and the scope of the invention will be indicated in theclaims.

Briefly, the recording medium of this invention is a plural layercrystallographically coherent sheet or tape structure which is very thinand flexible, yet strong so that it can be rolled up repeatedly on aspool or reel with substantially no degradation of the structure or theinformation stored thereon. In contrast to the media that characterizethe prior art, the present recording medium possesses a coherent crystalmorphology throughout; ideally it is a perfect hetero-epitaxially grownstructure. In its preferred form, the medium includes a thin, highlyflexible, optically clear, electrically insulating inorganicmonocrystalline base or substrate, a very thin, defect free, inorganicmonocrystalline light modulating section or modulator which includes aconducting zone for an electrode and a very thin, defect free, inorganicdual material storage layer sandwiched together to form a unitarystructure. The modulator is composed of an inorganic photoconductivematerial that is deposited on a defect free surface of the base usingthe base material as a "seed". The modulator material is compatible withthe base in that it has an atomic lattice that propagates the latticespacing of the base crystal with a nearly perfect match of atomicdistances so that it constitutes a nearly perfect heteroepitaxiallygrown layer on the base. Added to the modulator is the dual materialstorage layer composed of dielectric materials which continue that samecompatible crystal morphology and so maintains the atomic continuity andcoherence of the medium as a whole.

Thus, while the present medium has the same basic organization as someprior phototapes, i.e., base, photoconductive light modulating layer,and dielectric storage layer arranged in a sandwich, it is a whollyinorganic, coherent, primarily hetero-epitaxially grown crystalstructure. While its adjacent layers grow into one another for the depthof a few atoms, it has all of the vastly different electrical andoptical properties that such media require.

The medium incorporating my invention has several structural andoperational advantages, all of which are interdependent. Moreparticularly, the medium substrate or base is a monocrystal as noted,sapphire being the preferred base material When made with a high degreeof perfection, sapphire is exceptionally strong with a very lowcoefficient of thermal expansion, in the order of 5(10)⁻⁶ in./in./°C. Itis formed directly as a sheet or strip which is thin enough to be veryflexible and transparent, yet to be strong enough to provide a verydimensionally stable base for the medium's electrically active layersadded to it. Therefore, the base need not undergo grinding or otherdefect-producing treatment prior to receiving the other material layerscomprising the medium. Furthermore, the sapphire base is quite ruggedand abrasion resistant, as well as substantially unaffected by light,even at wavelengths at the ultraviolet end of the light spectrum.Accordingly, the base retains its strength, flexibility and opticalclarity over a very long period of time. Additionally, being inorganic,the base is not a source of outgassing contaminants when the medium isscanned electronically in a vacuum to read stored data from the medium.

In the present recording medium, the monocrystalline sapphire base notonly performs a supporting function as described, it actuallyestablishes the atomic arrangements of the remaining layers of therecording medium so that they all have very high degrees of perfectionas well. Rather than there being a distinct, uncontrolled andelectrically unpredictable physical boundary or interface betweenadjacent layers as in ordinary electrostatic recording media attemptedbefore, the adjacent layers of the present medium actually grow into oneanother. This arrangement provides a bond between the modulator and thebase which is very strong and which has substantially no defects, voids,etc. that could be a source of energy conversion losses and electricalnoise. Indeed, the modulator in its entirety has few defects, not onlybecause of its extreme thinness, but also because of its being formed byseeding from the base material itself. In other words, the atomiclattice structure of the modulator is quite ordered and defect free byvirtue of its having been established by nucleation sites on the defectfree surface of the very highly ordered monocrystalline base.

That same coherent morphology is continued into the dual materialdielectric storage layer superimposed on the modulator. This optimizesthe degree of perfection of the storage layer and the bonding of thatlayer to the modulator. Accordingly, the storage layer can be very thinso as to support a very intense electric charge field, and yet stillsuffer minimal charge loss due to leakage through that layer. Thus, itcan store an acquired optical or electrical image for a very long periodof time, i.e., several years, without degradation of that image. Ofspecial importance is the ability of the defect-free medium to store animage as unusually small, precisely defined charge domains in itsstorage layer. As we shall see, this gives the stored imagesexceptionally high resolution and enables the retrieval of those imagesby counting secondary electrons produced by electron beam scanning ofthe tape. This scanning process produces electrical picture signals withan equally high information content, while requiring a minimum voltageto power the beam during readout.

In summary, then, the plural layer medium of this invention is a whollyinorganic, crystalline, web-like structure which has exceptionally highsensitivity to light over a very broad spectral range from ultravioletto infrared. In one preferred embodiment, the medium is a very thin andflexible, yet strong tape. The light sensitivity and spectral responseof the medium are comparable to the very best silver halide photographicfilms. Therefore, it can acquire a useful optical image produced even byvery low-intensity light. Further, in having no mechanical boundariesbetween its various layers and in being substantially defect free,energy conversion losses in the medium are minimal and the medium givesrise to very little internal electronic noise. The optical signal thatis acquired by the medium is stored in the dielectric layer of themedium without requiring any chemical processing. Moreover, the mediumstores this data in a form that enables the stored image to be read fromthe medium by electronic scanning even after a prolonged period, i.e., ayear or more, as an electrical signal that can be processed or displayedusing conventional electronic circuitry. The medium can also be erasedand reused repeatedly without appreciable hysteresis loss. Thus, itpossesses the desirable characteristics of conventional magneticrecording media such as magnetic tapes and discs. Accordingly, themedium should find wide application wherever the accurate acquisition,longterm storage or display of high quality optical images or electronicpatterns is required.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description, taken inconnection with the accompanying drawings, in which:

FIG. 1 is a fragmentary diagrammatic view in cross-section of anoptoelectronic signal recording medium embodying the principles of thisinvention;

FIG. 2 is a diagrammatic view of apparatus for making the FIG. 1 mediumso as to have a coherent crystal morphology;

FIG. 3 is a graphical diagram showing the spooling characteristic of theFIG. 1 medium formed as a tape;

FIG. 4 is a similar diagram showing the bending stress characteristic ofsuch a tape; and

FIG. 5 is an isometric view of an interactive electronic image recordingsystem embodying the invention implemented as a microscope;

FIG. 6 is a sectional view on a larger scale taken along line 6--6 ofFIG. 5;

FIG. 7 is a fragmentary isometric view on a still larger scale showingthe recording medium or tape used in the FIG. 5 system;

FIG. 7A is a similar view showing a portion of the FIG. 5 instrument ingreater detail;

FIG. 8 is a sectional view on an even larger scale taken along line 8--8of FIG. 7;

FIG. 9 is a view similar to FIG. 7 showing a portion of the FIGS. 5 and6 system in greater detail;

FIG. 10 is a side elevational view on a larger scale taken along line10--10 of FIG. 9.

FIG. 11 is a diagrammatic view illustrating the exposure of the FIG. 7medium.

FIG. 12 is a graph showing the mode of controlling exposure.

FIG. 13 is a view similar to FIG. 11 which helps to explain the removalof excess charge of the FIG. 7 medium;

FIG. 14 is a graphical diagram that also helps to explain that step; and

FIG. 15 is a view similar to FIGS. 11 and 13 showing the electronicimage stored on the FIG. 7 medium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 of the drawings, the medium of this invention,indicated generally at 34a, is a thin, flexible, plural-layer, inorganicweb structure of indeterminate extent. The thicknesses of its variouslayers have been exaggerated in FIG. 1 for ease of illustration. Usuallythe medium takes the form of a long tape or ribbon; but it could beformed as a disk or drum by mounting it to a stiff backing. Also, eventhough medium 10 is a plural layer structure, it has a coherent crystalmorphology. In other words, although the different layers are sharplydelineated in FIG. 1, as we shall see, as far as their crystal latticesare concerned, there are no such sharp boundaries or borders inactuality between adjacent layers; rather, they grow into each other atleast to the depth of a few atoms.

Medium 10 comprises a substrate or base 102, a light modulator 104 and adual-material dielectric storage layer all superimposed and grown toform a unitary structure. Thus, although at first glance medium 34aseems to be similar to the phototapes and films described in theabove-identified prior publications, particularly Hutter et al, inactuality it represents a radical structural departure from those priorelectrostatic recording media and it, therefore, yields hithertounattainable performance properties. These, in turn, make possible andpractical an entirely new type of optoelectronic recording system.

The base 102 of medium 34a is thin, transparent and formed as a singlecrystal of an inorganic material, preferably sapphire. Although layer102 is only a few, e.g., 5, microns thick, the rhombohedral structure ofits aluminum and oxygen atoms provides exceptionally high cohesivestrength without forming easy cleavage planes, unlike glass and othersilicon or carbon-based materials which constitute the substrates ofsome prior rigid electrostatic recording media. As a result, base 102has exceptionally high mechanical strength and dimensional stability.Also the crystalline arrangement of layer 102 is such that its c-axis isoriented perpendicular to he basal plane of the layer so that thesurfaces of that layer are unusually smooth, free of defects and arescratch resistant. Furthermore, being formed as an ultrathinmonocrystal, layer 102 is as flexible and spoolable as the plasticsubstrates used in conventional magnetic tape, yet it is optically clearover a substantially wider wavelength spectrum then are the bases andsubstrates used in prior recording media of this general type.

Another very important reason for forming base 102 as a sapphiremonocrystal is that sapphire has a rhombohedral crystal structure whoselattice is consistent with, and whose atomic spacing more or lessmatches, those of some photoconductive and dielectric materials that canbe used to form the remaining layers of medium 34a. In the medium 34aspecifically illustrated herein, the modulator 104 is composed ofsilicon which has a diamond-cubic crystal arrangement with an atomicspacing that matches closely the oxygen lattice periodicity of thesapphire.

Accordingly, when the different zones or layers comprising modulator 104are deposited on base 102 by R.F. sputtering or other known means undercontrolled conditions to be described, the base material functions as a"seed" crystal. That is, it provides nucleation sites for the silicon sothat the modulator 104 is added to the base as a monocrystal whichpropagates the lattice spacing of the sapphire. In other words, themodulator 104 materials are encouraged, if not compelled, to assume anatomic arrangement compatible with the sapphire morphology so that thebase and modulator materials actually grow into one another where theymeet. Thus, although the modulator 104 is different chemically andelectrically from base 102, those parts of the medium share the sameatomic lattice at their interface so that there is an unusually strongcoherent bond between the two. Also, as noted above, the monocrystallinebase 102 is made free of defects. Furthermore, no defect-producinggrinding or finishing is involved in the creation of the ultra thin base102. Therefore, the base receives the added-on layers comprising themedium without introducing defects into these added-on layers.Accordingly, the modulator 104 that is deposited on base 102 is equallydevoid of defects that could increase energy conversion losses when anoptical image is recorded on the medium or result in the recording of adistorted image.

Still referring to FIG. 1, in order to facilitate the recording of anoptical image on the medium, modulator 104 includes a conductive layeror zone 104a adjacent base 102 to provide an electrode at that location.Layer 104a is simply a zone of modulator 104 containing an n-type dopantof phosphorous atoms or the like added to the silicon during the initialdeposition of the modulator 104 on base 102. This n-type layer or zonewhich is only in the order of 0.2 micron or less thick, has the samecoherent atomic lattice structure as the modulator 104 as a whole. Theremaining zone 104b of the modulator is devoid of dopants. During theexposure process to be described later, electron-hole pairs are producedin modulator zone 14b by the photons incident on medium 34a. Electronsmove to zone 104a and these charges are neutralized in proportion to theabsorbed light, while the positive carriers or holes are captured by thestorage layer 106.

Dual material storage layer 106 is added to modulator 104 by chemicalreaction in a conventional sputtering or coating process so as to havethe same coherent crystal morphology as the modulator. In medium 34a,layer 106 is composed of silicon nitride (Si₃ N₄). Other suitablematerials include sapphire (Al₂ O₃). These inorganic materials havehexagonal crystal structures with atomic spacings which match well thespacing of the silicon atoms in modulator 104. Layer 106, being actuallygrown from nucleation sites on the smooth surface of modulator 104 isfirmly bonded to the modulator and is highly ordered with few internaldefects, giving layer 106 great signal storage capability. In otherwords, even though layer 106 is quite thin, in the order of 0.2 micron,positive and negative charge carriers can reside on opposite sides ofthat layer with minimal measurable long-term charge loss through thatlayer. Preferably, storage layer 106 includes a storage zone 106a and anultrathin (i.e., 10 Angstrom) interfacial zone 106b of a dielectricmaterial adjacent the modulator 104. A suitable such material is silicondioxide (SiO₂). This zone 106b, which is anisotropic, inhibits lateralcharge migration or conduction in the plane of layer 106, therebypreventing loss of resolution of the stored image over time. Theremaining zone 106a is pure silicon nitride.

The thinness and perfection of storage layer 106 enables animage-representing electric field distribution to be maintained by themedium that is 100 times more intense than that tolerated by priormedia, even though medium 10 is in the order of 100 times thinner thanthose prior structures. During the recording process, the storage layer106 can accept and store exceptionally uniformly distributed chargesover wide areas of the medium in the form of minute charge domains. Thisenables the medium to locally record and store minutely differentoptoelectronic signals on a background of minimal noise. Resultantly,optical or electronic signals of very high resolution can be recordedand stored even under poor lighting conditions. By the same token, as weshall see, the image-representing charge pattern can be scanned by anelectron beam maintained at a relatively low voltage to produce apicture signal that has the same very high information content as thestored image. Its ability to store an electronic pattern across itsdielectric extent and to maintain high electric fields across itsthickness without dielectric breakdown and/or slow leakage permitsmedium 34a to store signals for an extended period of time, even inlighted conditions. A typical medium 34a incorporating this inventionhas the following overall characteristics:

    ______________________________________                                        Detectability Threshold                                                                          5 electrons (or photons)                                   Quantum Efficiency 1 (100%)                                                   Signal-to-Noise Ratio                                                                            3000:1 (3 sec.)                                            Tensile Strength   up to 40,000 kg/cm.sup.2                                   Modulus of Elasticity                                                                            4 × 10.sup.6                                         ______________________________________                                    

Additionally, as shown in FIG. 3, if medium 10 is formed as a tape orribbon with a thickness of about 6 microns and a length of 400 cm, itwill occupy relatively little space when spooled. For example, a coil oftape wound on a 5 mm core would have a diameter of only 7.5 mm. FIG. 4shows the bending stress on such tapes when spooled. As shown there, thestress on the 6 microns thick tape in the above example is about 4.2kg/cm² at the inner end of the tape reducing to only 1.4 kg/cm² at theouter end of the tape. As seen above, the elasticity and high tensilestrength of the medium's sapphire base 102 enables the medium towithstand such stress quite easily.

In the production of medium 34a, base 102 may be formed using acombination of the meniscus techniques used in wet emulsion coating andthin film coating techniques wherein crystals are drawn in edge-fedgrowing processes from a melt. A principle advantage of growing base 102in this fashion is the ability to produce the base at an appreciablerate as a relatively wide, ultrathin web having not only an essentiallyperfect crystal morphology, but also with mirror smooth surfaces whichneed not be cut, ground, polished or otherwise finished as required withother substrates to their detriment as noted above. Because theresultant web possesses flexibility as a desired virtue due to itsthinness, it can be rolled, slit and otherwise processed into thedesired form as it emerges from the crystal growing apparatus. Itstensile strength is many times that of steel, while it also canwithstand temperatures well above those of steel.

A preferred technique for making a plural layer hetero-epitaxially grownweb such as medium 34a is described in detail in applicant's copendingapplication Ser. No. 872,893 of even date herewith, entitled METHOD ANDAPPARATUS FOR MAKING INORGANIC WEBS AND STRUCTURES FORMED THEREOF, andthat disclosure is incorporated herein by reference. That technique willbe described briefly at this point with reference to drawing FIG. 2 tocomplete this description. As shown in that drawing figure, the growingapparatus indicated at 2 includes a molybdenum or iridium vessel 3containing a melt M of the substrate 102 material, namely sapphire (Al₂O₃), which is maintained at a temperature in excess of 2000° C. Thesurface of this melt M is caused to touch a smoothly polished molybdenumor iridium drum 4 which revolves above the melt bath at a spacing whichallows merely a meniscus connection 5 between the surface of the meltand the wettable drum surface 4a. The geometry of the neck 5 providesthe mechanism for controlling the thickness of the melt that will bedeposited on the drum's surface 4a as the drum is rotated in a clockwisedirection.

Crystal growth is established on the drum by initially dipping the drum4 into the melt M and then withdrawing the drum until only the thin neck5 connects the drum surface to the melt. As the drum rotates clockwise,it draws liquid from the vessel 3, thus progressively coating the drumsurface 4a with melt material M which spreads as a thin film or coating102a of uniform thickness to the ends of the drum. The thinness of neck5 determines the thickness of the liquid film 102a applied to the drumsurface. That, in turn, is dependent upon the viscosity and density ofthe melt, the wettability of the drum surface 4a measured by the angleof contact between neck 5 and that surface, the spacing of the melt Msurface and the drum and, finally, the drum surface velocity.

The liquid film 102a deposited on the drum surface 4a revolves with thatsurface, being cooled by cooling coils 6. When its leading edge arrivesat a chilling station 7, that leading edge makes contact with the coldedge of a sapphire monocrystal seeding bar 8 whose internal crystalorientation represents the crystal structure desired for base layer 102.That cold contact solidifies the film 102a and as the film solidifies,it propagates the arrangement of atoms found at the edge of the seedingbar 8. As described in the above application, the interaction of eventsat the station 7 demands close control over the thermal gradients in theprogression of the solidification of the liquid film both towards thedrum surface 4a , as well as in the circumferential direction around thedrum. As the film starts to solidify, with its undersurface stillliquid, it is peeled from the drum surface 4a at a stripping station 9as a single crystal sapphire web 102.

That web which will constitute the base 102 of medium 34a is conductedinto a heated oven 11 maintained in the same environment as apparatus 2.As the sapphire base 102 traverses a first section 11a of the oven, itis maintained at a temperature in the order of 1,000° C. or less. Also,that web is exposed to a gas stream from a gas source 13a. This gas,typically SiH₂ Cl₂, doped with phosphorous atoms decomposes at thattemperature and, in a sputtering process, builds a layer 104a of n-dopedsilicon on layer 102 from nucleation sites on layer 102, the siliconassuming the atomic arrangement described above. The web now consistingof layer 102 and zone 104a passes into oven section 11b which receives,via a pipe 13b, the same basic gas without any additive or dopant toform a photoconductive silicon zone 104b which completes the medium'smodulator 104.

From oven section 11c, the moving web passes into a third oven section11a which receives, via pipe 13c, a mixture of SiH₄ +NH₃ and oxygengasses. In this oven section, the web is maintained at a temperature inthe order of 800° C. or less which causes the disassociation of that gasmixture. That, in turn, brings about the growth on zone 14b of acrystalline silicon dioxide zone 106b. Finally, the web proceeds throughoven section 11d into which the same gas without oxygen issues through apipe 13d. This deposits a crystalline silicon nitride zone 106a on theweb which completes the medium's storage layer 106. While zone 106agrows in thickness, it is subjected to simultaneous bombardment from anelectron beam source 15 in oven section 11d which optimizes the degreeof perfection of layer 106a by minimizing crystal lattice defects in thegrowth process. The web emerges from oven section 11d and is cooled,thereby completing the formation of medium 34a. After cooling, thecompleted web 10 can be slit or otherwise cut to tape or sheet form asdesired.

The medium 34a described herein and made as aforesaid avoids thedisadvantages that characterize prior multi-layer electrographic filmsand phototapes by incorporating a coherent crystal morphology throughoutthe medium through repetition of similar atomic distances in all of itslayers beginning with the monocrystalline base 102 to create a unitaryhetero-epitaxially grown structure. Being of exceptional purity andoptical clarity and having no sharp internal mechanical boundariesbetween layers, the medium operates on a much higher performance planewith respect to its light sensitivity and spectral response than priormedia of this general type.

This medium represents not simply an improvement over previousstructures, but rather it establishes an utterly new class ofsupersensitive materials for optoelectronic signal recording which arecapable of acquiring and storing optical data at sensitivity levels andat densities previously unattainable.

Indeed, the fact that the properties and performance of the presentmedium are so superior to the prior films discussed above has madepossible the development of an entirely new type of read/write system orcamera designed around this medium.

FIGS. 5 and 6 show such a system which, for purposes of thisdescription, takes the form of a microscope-camera 10 capable ofacquiring and storing electronic images of very small specimens orobjects. However, the invention could just as well be implemented as adifferent type of recorder, such as a camera, by substituting theappropriate camera optics or lens system.

The microscope 10 comprises a rigid housing 12 which is supported by astand 14 above a standard X-Y-Z slide table or positioner 16 mounted toa pedestal 18 projecting up from the base of the stand. The positioner16 is arranged to support and position a glass slide G on which thespecimen S to be viewed is placed. Using the positioner 16, the specimenS can be spotted on the viewing axis A of the microscope 10. Aftermicroscope-camera 12 takes a picture of specimen S, which is stored onan optoelectronic recording medium 34 (FIG. 6) inside the microscope,that apparatus can be operated in a readout mode to retrieve the storedimage for display or reproduction using a CRT/printer unit indicatedgenerally at 20 connected electrically to the microscope by a cable 21.

As best seen in FIG. 6, the microscope housing 12 is divided into aplurality of internal compartments. More particularly, there is a tapetransport compartment 22 at the bottom of the housing which contains apair of rotary spindles 24 and 26 for supporting take-up and let-offspools or reels 28 and 32, respectively, between which stretches theoptoelectronic recording medium which is in the form of a long phototape34. When the spindles 24 and 26 are rotated, the tape is advanced alonga focal plane indicated generally at P which constitutes the exposureposition of the tape.

The bottom wall of housing 12 is formed with a generally cylindricalcavity 35 which intercepts compartment 22 directly opposite plane P. Theinner end of that cavity is closed by a transparent glass platen 36 thatisolates compartment 22 from cavity 35. While in FIG. 2 the platen 36 isshown separated from the tape, in actuality, its surface 36a positionsthe tape at focal plane P. The platen may also constitute an opticalelement in the microscope's optical path to reduce field flattening,color correction, filtering, etc. of the incoming optical image.Furthermore, as we shall see, the platen has special light sensingcapabilities that are used to focus the microscope automatically priorto taking a picture and to set the exposure duration when the picture isbeing taken.

The camera's movable lens unit, indicated generally at 38, is rotativelymounted in cavity 35 and the microscope is focused onto specimens bycontrolling a servomotor 39 that moves the lens unit axially veryprecisely in one direction or the other. Of course, the instrument canalso be focused manually by appropriately moving unit 38.

The tape 34 is moved back and forth between the two spools 28 and 32 byreversible servomotors 42 which rotate spindles 24 and 26 respectively.By applying currents to these motors 42 of the appropriate polarities,the tape 34 may be kept taut and moved in either direction to position aselected tape frame on platen 36 at the microscope's focal plane P. Insome applications, the tape may be advanced by other means such as acapstan or a linear or eddy current motor using a metallized margin ofthe tape itself.

The mechanism for transporting tape 34 may include other components,such as tape edge guides and a tape gate for actually locating eachincrement or frame of the tape at an exposure position in the imageplane P. However, for ease of illustration these components, which arefound in many conventional automatic cameras, have not been shown in thedrawing figures

Microscope 10 includes another compartment 46 which contains thecamera's control section 48. That section includes a microprocessor andcurrent drivers for providing the drive signals for the drive motors 42and for the camera's gate (if present). The makeup of section 48 and theprogramming of its processor will be obvious from the control functionsto be described. When the operator pushes a recessed FORWARD button 50(FIG. 1) in the side wall of housing 12, the control section 48 willapply a selected number of pulses to motors 42 to shift the next tapeincrement or frame into the exposure position at the image plane P.Signals from control section 48 to the motors will shift the tape frameby frame in the opposite direction when a recessed REVERSE button 51 onthe side of the housing 12 is depressed. Preferably, buttons 50 and 51and the camera's other control buttons to be described are capacitive"touch" buttons built right into the wall of housing 12. These othercontrol buttons include a FOCUS button 49 which may be depressed toautomatically focus instrument 10, an EXPOSE button 52 which initiatesthe recording of an optical signal on the tape 34, a READ button 53which initiates a read operation on the tape to produce picture signalscorresponding to an image stored on the tape and an ERASE button 54which is depressed to erase an image already stored on the tape inmicroscope-camera 10. Also, a tape frame counter 55 is mounted in thetop wall of housing 12.

The power for motors 42 and for control section 48 and the otherelectromechanical parts of the apparatus derives from a power supply 56,including batteries, contained in a compartment 58 of housing 12 locatedabove compartment 46. Appropriate electrical conductors are providedbetween these parts as wires or printed circuits inside the housing.Access to the interior of the battery compartment 58 is had by removinga small cover 12a (FIG. 5) in the front wall of housing 12. Preferablyalso, the batteries in the power supply 56 are of the type that can berecharged by connecting them to a source of DC power by means of afemale connector 62 located at the bottom of stand 14 as shown in FIG.5.

Housing 12 also has a large compartment 64 which is aligned with theaxis of lens unit 38, which axis coincides with the optical axis A ofthe microscope. Compartment 64 contains the various stationary lenses 66that comprise the microscope. These are all centered on axis A and theoperator uses the microscope to observe specimen S by looking through aneyepiece 68 in the top wall of housing 12.

Referring now to FIG. 6, microscope-camera 10 also includes a fieldemission device or electron source 74 which is slidably mounted in thehousing just above platen 36. The source can be moved between anextended position shown in solid lines in that figure wherein itoverlies the tape frame at the focal plane P and a retracted positionshown in dotted lines in that same figure in which the gun is located inhousing compartment 46 away from the tape. While source 74 may beshifted between its two positions by any suitable means, in theillustrated apparatus, it is moved by a servomotor 78 located incompartment 46 and coupled to source 74 by way of a rack and pinionarrangement. The electron source 74 is normally located in its retractedposition so that it does not obstruct the operator's view through themicroscope. However, during the exposure process, the source is moved toits extended position overlying the tape by motor 78 under the controlof section 4. Section 48 then causes source 74 to direct a cloud ofelectrons from discharge points 74a of source 74 against the upper sideof the tape frame present at the focal plane P. As we shall see, theupper surface of the tape frame at plane P becomes charged with thesenegative carriers, enabling that frame to acquire and store anelectronic image corresponding to the optical image projected onto thatframe by the instrument's lens unit 38. The amount of the charge iscontrolled in terms of time and magnitude to assure the capture of themaximum amount of information contained in the image to be recorded. Aswe shall see, the electron source 74 is also used to eliminate theelectrical bias field from each tape frame after the exposure of thatframe by removing excess charge carriers from the frame.

Microscope-camera 10 also includes an electron gun 84 located in a largehousing compartment 86 to the left of compartment 64 and used wheninstrument 10 is operated in its read-out mode. Unlike source 74,electron gun 84 directs a finely focused beam of electrons to theexposed tape frame present at a read plane or position R in compartment86 that is defined by the bottom wall of that compartment. Gun 84 iscontrolled so that the electron beam sweeps out a raster on the uppersurface of that frame by a circuit 88 located in a housing compartment92 positioned just to the left of compartment 86. Since tape 34 istemperature dependent, preferably the gun is a cold cathode device thatdoes not generate heat.

During read-out, the scanning electron beam from gun 84 causes secondaryelectrons to be emitted from the tape frame being scanned whosenumerical distribution by area elements (pixels) represents theelectronic image stored on that frame. These secondary electrons arecollected by an annular electron collector 94 located near the top ofcompartment 86 which thereupon produces a signal which is the electricalanalog of the stored image. That signal is applied to a read-out circuit96 contained in a housing compartment 98 to the right of compartment 86where it is amplified, digitized and otherwise conditioned before beingapplied to the various conductors of the connector 21a to which cable 21is coupled as shown in FIG. 1. Those picture signals are then fed by wayof that cable to terminal 20 where the retrieved image can be viewed orreproduced.

In the microscope-camera 10 specifically illustrated herein, the sametape 34 is intended to remain permanently in the housing compartment 22.Accordingly, that compartment, along with compartments 46, 86 and theportion of compartment 64 below the lowest lens 66, is maintained undera high vacuum, in the order of 10⁻⁸ Torr. To hold the vacuum, airtightseals (not shown) are provided between platen 36 and the wall of cavity35 and between the lowest lens 66 and the wall of compartment 64. Thesecompartments are thus free of dust, moisture and other contaminants thatcould interfere with the electrons from the electron sources 74 and 84.

Refer now to FIGS. 7 and 8 which show the optoelectronic tape 34 ingreater detail. It is composed of a large number of imaging segments orframes 34a and an equal number of viewing segments or frames 34b whichalternate along the length of the tape. The tape is made in toto ofinorganic materials, as opposed to organic plastic materials. Therefore,it does not produce dreaded contamination caused by outgassing in thehigh vacuum environment of the microscope and it will, therefore, notproduce any adverse effects on the electrons emmitted from guns 74 and84.

Basically, the tape is a unitary hetero-epitaxially grown structurecomprising a flexible, optically clear (from 0.2 to 5.0 micrometers)ribbon-like monocrystalline sapphire (Al₂ O₃) base or substrate 102.Added to base 102 in each imaging area 34a of the strip are a thin(i.e., about 10,000 Å) modulator 104 composed of a photoconductivematerial, such as silicon (Si) or gallium arsenide (GaAs), and a verythin (i.e., 1,000 Å) dual-material storage layer 106. A very thinphosphorus-doped zone 104a of modulator 104 (i.e., n-doped with fixedpositive charges) is present adjacent base 102 to serve as an electrode.The remaining zone 104b of modulator 104 is free of additives.

The dual-material storage layer 106 is composed of a very thin (i.e.,about 1,000 Å) storage zone or layer 106a made of a suitable dielectricmaterial such as silicon nitride (Si₃ N₄) and an ultra- thin (i.e.,about 30 Å) interfacial zone 106b of an anisotropic dielectric materialsuch as silicon dioxide (SiO₂) at the underside of zone 106a. Zone 106bexhibits electrical insulating behavior that prevents penetration ofthermally generated or even photogenerated charge carriers in modulator104 to the undersurface of storage zone 106a; but zone 106b does allowtunnelling through to the storage zone 106a of photogenerated chargecarriers under the influence of a suitable superimposed strongorthogonal electrical field through the tape layers 104a, 104b, 106b and106a. In other words, charge carriers from the modulator 104 that havetunnelled through zone 106b under the influence of an applied field are"pinned" to the underside 106c of storage zone 106a in so-called chargecentroids. In the absence of that field, zone 106b prevents additionalcarriers from reaching the storage zone and disturbing the properlyaccumulated charge count there. Thus, zone 106b traps all photogeneratedpositive carriers created during the exposure step in storage zone 106a,thereby storing an electronic signal pattern spatially in that zone ofthe tape and preventing also any lateral movements of said chargecarriers in zone 106a so that an image having exceptional resolution ismaintained for many years.

The tape 34 is very thin, being only a few microns thick, so that it isflexible enough to be wound easily onto reels 28 and 32. It may be made,for example, by the process described in applicant's copending patentapplication of even date herewith entitled Method and Apparatus ForMaking Inorganic Webs and Structures Formed Thereof, which disclosure isincorporated by reference herein. The imaging areas 34a of the tape haveextraordinary properties, among which are extremely high sensitivity orphotospeed, comparible to a silver halide film speed in the order of ASA3,000. Each of these areas is imageable at low energy levels (e.g. 20electrons minimum/pixel) due to low inherent noise (defects) and darkcurrents (threshold minimums). Thus, each of the areas has the capacityto acquire a very high quality electronic image corresponding to theoptical image projected onto it by the microscope's lens unit 38.Furthermore, because of the barrier and trapping functions of the tape'sunique dual-material storage layer 106, an image can be stored on thetape areas 34a for several years without any appreciable degradation ofthat image.

The images stored on the tape frames 34a can be read by scanning thesurfaces 106d of those areas using the electron beam from gun 84 toproduce exceptionally high quality displays or reproductions of thestored images. If desired, the image on each tape frame 34a can beerased by exposing the frame to ultraviolet light from a U.V. lamp 110(FIG. 6) mounted in housing compartment 64 just above tape 34. Thisradiation discharges the frame's dielectric layer 106 enabling the filmframe to be reused repeatedly and the frame area does not lose itsoptical signal acquisition and storage capabilities with such repeatedusage.

The base or substrate 102 of tape 34 is quite transparent so that thesegments of that substrate in the viewing frames 34b of the tapeconstitute windows. When one of these frames is located at themicroscope's focal plane P, the operator sighting through eyepiece 68can see right through that frame to the object being viewed, i.e.,specimen S (FIG. 5).

In another application, as when the tape 34 is processed in a singlelens reflex camera incorporating my invention, the surface of thesubstrate 102 may be abraded, etched or otherwise treated in the tapeframes 34b so that it has the characteristics of frosted glass. If themodulator 104 and storage layer 106 are etched away to form the viewingframes, then only the clear sapphire substrate remains in the opticalpath for viewing the scenery as through a telescope; however, therefractive index of the substrate must be considered when the additionallens elements are calculated for the viewfinder subsystem. In any event,a virtual image of the scene in the camera's field of view will beprojected onto the viewing frame located at the camera's focal plane andthat image can be seen from behind the frame by looking through thecamera's viewfinder eyepiece. It should be noted that the red, green,and blue filter lines, which represent the primary colors, will appearas white to the viewer (daylight spectrum). Also, of course, thephototape may consist entirely of imaging frames for use with aninstrument having a seperate viewfinder.

Referring to FIG. 7, proper exposure of the imaging frames 34a of thetape requires that voltages from power supply 56 be applied to theconductive zone 104a at those frames. Accordingly, in the forward edgemargin of each imaging area, the material zones 106a, 106b, and 104a to104b are etched away so that a conductive strip 112 can be laid down onconductive zone 104a. If desired, in some applications the strip 112adjacent each frame 34a may be isolated electrically from the similarstrips associated with the other frames of tape 34 so that electricalconnections may be made to each frame independently. As shown in FIGS. 6and 7, when a particular imaging frame 34a is present at themicroscope's focal plane P, a contact finger 118 at the front ofcompartment 64 contacts strip 112. As shown in FIGS. 2 and 4, thatcontact finger is connected in parallel to switches 122 and 124 in themicroscope's control section 48. Alternatively, electrical connectionsto the strip may be made through the spool spindle 24 or 26.

As best seen in FIG. 6, an optical detector 134 connected to controlsection 48 is located at the righthand corner of compartment 64 abovethe tape. It is arranged to detect the transition from a transparentviewing frame 34b to the next opaque imaging frame 34a, i.e., theleading edge of an imaging frame. Whenever section 48 receives adetector 134 signal, it indicates that a viewing frame is positionedproperly at focal plane P. That signal also indicates that the previousimaging frame 34a (or the tape leader) is located at the readout plane Rin compartment 86 for a read operation on that frame by electron gun 84.A second similar optical detector 136 is positioned just above the tapeon the righthand wall of compartment 86. Detector 136 emits a signal tosection 48 whenever it detects the leading edge of a frame 34a, thusindicating that a frame 34a is positioned properly at focal plane P,ready for imaging. Thus, the detectors 134 and 136 together provideposition signals to section 48 enabling that section to controlservomotors 42 to position a tape frame 34a or 34b at either theexposure position at focal plane P or the scanning or readout positionat readout plane R.

Refer now to FIGS. 9 and 10 of the drawings which depict the portions ofmicroscope-camera 10 that set automatically the instrument's focus andits exposure in accordance with the prevailing lighting conditions.These parts include an array of numerous thin, parallel, transparent,abutting, bandwidth-limited, electrically insulating, color filterstripes 142 formed on the platen surface 36a that supports the tape 34at the camera's focal plane P. The stripes extend longitudinally andparallel with respect to the tape edge so that they coincide with thescan line pattern associated with the electron beam from gun 84.Although the drawing figures illustrate stripes 142 as being relativelythick and few in number, in actuality there may be several thousandstripes in the array on platen 36, each stripe being in the order ofonly a few microns wide and a few microns thick.

The filter stripes 142 on platen 36 consist of very fine abuttingparallel red (R), green (G), and blue (B) films which divide theincoming light image into its color components. Thus, when a tape frame34a is exposed at plane P, the image applied to the frame consists ofred, green and blue color components of the object being viewed whichare interlaced on the frame as shown. In other words, the pictureinformation for each color component of the picture is stored everythird line on the tape frame. The color filter lines coincide with theraster path of the scanning electron beam from gun 84, when that imagingframe is located at read-out plane R in compartment 86. The width of thescanning electron beam may be slightly less than the width of the filterstripes to compensate for any residual skew and any minutemisregistration of the tape frame 34a between its exposure position atplane P and its read-out at plane R.

Interspersed with the stripes 142 are a series of thin, photoconductivestripes 143 featuring large band width sensitivity. The function ofstripes 143 is to detect incident light levels when their photo-currentsare all integrated and image contrast (focus) when their differentialphoto-currents attain the widest amplitude spread. Suitablephotoconductive materials for stripes 143 include silicon or galliumarsenide (GaAs). Electrical leads 144a and 144b lead from the conductivelayers of each stripe 143 to the camera's control section 48. The numberof photoconductive stripes 143 may be only 10 or 100 out of the severalthousand filter stripes 142, placed at ninety line intervals, forexample. When a voltage is applied across each stripe 143, the currentthrough that stripe will provide a measure of the intensity of the lightincident on that stripe. The photodetector stripes 143 are quite opaqueas compared to the color filter stripes whose transparency exceeds 90%in the bandwidth limited region but since they are relatively few innumber, they attenuate the incident light only minimally.

Preferably, a transparent conductive film or layer 145 overlies stripes142 and 143, clearing the latter as shown in FIG. 10, to form anelectrode which is connected by a lead 145a to control section 48.During the exposure process, control section 48 biases layer 145negative with respect to the tape conductive layer so that frame iselectrostatically attracted to platen 36 and held closely to the filterstripes 142. On the other hand, when the tape is being moved before andafter exposure, section 48 applies a DC voltage of the opposite polarityto layer 145 so that the tape is electrostatically repelled from theplaten 36 to minimize scratching of the tape base 102.

When the operator depresses the EXPOSE button 52 (FIG. 5) to record anoptical signal on a tape frame 34a just prior to exposure of that frame,control section 48 connects stripes 142 to the power supply 56 so that aconstant voltage is applied in parallel across all of the photosensitivestripes 143. The control section then samples and integrates thecurrents through the stripes to develop a total flux (TF) signal whichrepresents the total light flux incident on the tape frame 34a beingexposed. That TF signal is then used by control section 48 to controlthe charging current flowing during the separately computed on-time ofthe electron source 74 during the exposure process; the control sectionalso "finds" the stripe producing the smallest signal, representing thedarkest part of the image, the magnitude of that signal, referred toherein as the exposure duration (ED) signal, being used by section 48 tocontrol the "on time" of the electron source 74 during the exposureprocess, the mathematical product of current and "on time" beingproportional to the incident light flux.

Refer now to FIG. 11 which shows the electrical environment of the filmframe 34a during exposure and FIG. 12 which depicts a typicalcharacteristic curve C for the frame being exposed. Effectively, controlsection 48 controls a variable resistor 146 connected in series withelectron source 74, a 5-100 volt tap of power supply 56, switch 122 andtape imaging frame 34a at layer 104a thereof so that the darkest part ofthe light image being projected onto the frame receives a selectedminimum exposure, i.e., at least 10⁹ photons/cm² corresponding 10⁻³ergs/cm². In a typical case, the charging current in the FIG. 11 circuitis under one ampere and persists for one microsecond to one second (ormore), depending upon the amount of light incident on the tape. Eachincident photon produces one electron-hole pair in modulator layer 104as shown in FIG. 11. In the portions of modulator 104 where the lightimage is darkest, the incident photons emanating from a faint imagetypically produce in the order of 3·10⁸ electrons/cm². For the brightestparts of the modulator, there may be in the order of 3·10¹¹photogenerated electrons/cm². Thus, the charges stored at differentlocations on layer 106 may vary from, say, 20 electrons/pixel to 20·10³electrons/pixel. The difference yields a dynamic range of 1000:1,permitting the retrieval of far more than the desired thirty twodifferent grey levels G in the image being recorded on the tape frame34a, as shown in FIG. 12.

The electric field across the tape causes the photogenerated electronsto move toward conductive layers 104a from where they are conducted awayto the ground plate of the battery 56 via conductive layer 104a. Thephotogenerated positive carriers or holes move toward tape storage layer106. Under the influence of the strong superimposed external fieldextending between the electrode layer 104a and the virtual electrodeformed by electron deposition on surface 106d and the additionalinternal field formed between negative electronic charges on the surface106d of layer 106 and the innate positive potential of the holes, theseholes tunnel through the interfacial zone 106b and are trapped in theundersurface 106c of the dielectric zone 106a in numbers that are indirect proportion to the image brightness in the different parts of theimage area I of the frame 34a. These positive charges are balanced byequal numbers of electrons from source 74 that repose on the surface106d of layer 106 as shown in FIG. 11. Although the charge domains ornumbers of electrons stored at adjacent pixels on tape surface 106d mayvary to establish the contrast or grey levels in the stored electronicimages, the potential versus electrical ground is equalized throughoutthe frame area. Thus, during exposure, control section 48 charges frame34a to a voltage and for a time so as to operate on the optimum segmentof the tape's characteristic curve C (FIG. 8) under the prevailinglighting conditions. Accordingly, there is no possibility ofover-exposure or under-exposure of the picture being taken by camera 10and stored on each tape frame 34a in an exposure energy range from aminimun of 10⁻³ ergs/cm² to 10 ergs/cm².

As noted above, the photosensitive stripes are also used to focus thecamera when a viewing frame 34b is located in the focal plane P.Accordingly, the specimen S (FIG. 6) will assuredly be in focus whenseen through eyepiece 68 and frame 34b or when photographed on the nextimaging frame of the tape. More particularly, when control section 48receives a signal from detector 134 indicating that a viewing frame 34bis positioned at focal plane P, it provides a constant voltage acrossstripes 143 and samples the current signals from these stripes asdescribed above. When an out-of-focus image is projected onto the arrayof stripes which, in fact, defines the camera's focal plane P, thatimage will be blurred and will have little or no gray leveldifferentiation or contrast over the image area in plane P. Accordingly,the output signals from the array of stripes 143 will have acorresponding lack of differentiation. As the projected image at plane Pis brought into focus, there is greater contrast between light and darkareas of the projected image. Ultimately, when the image projected ontothe stripe array is in exact focus, the differences between the lighterand darker areas of the image will reach a maximum, as will theamplitude spread of the differential photo currents from the stripes 143corresponding to those image areas.

During the focusing process, control section 48 repeatedly samples theset of signals produced by the stripe array. During each such sampling,after being digitized, the signals from the stripes are subtracted todevelop a set of difference signals which are averaged and inverted toproduce a feedback signal to control the motor 39 that moves lens unit38. If, as a result of a given sampling, the motor 39 is driven toimprove the focus, the feedback or difference signal resulting from thesubsequent sampling of the stripe signals will reflect that fact and thedriving of the motor 39 will continue until the feedback signal isreduced to zero. On the other hand, if there is no improvement in thefocus after a few samplings and consequent lessening of the feedbacksignal, indicating that the lens unit 38 is being moved in the wrongdirection to achieve focus, control section 48 will reverse the polarityof the voltage applied to motor 39 so that during subsequent samplingsof the stripe 143 array, the resultant feedback signal will cause motor39 to move unit 38 in the right direction to focus the microscope-camera10.

The automatic focus procedure described above is initiated just prior toexposure by control section 48 following depression of EXPOSE button 52.It can also be initiated by depressing the FOCUS button 49 on housing 12if a specimen is to be viewed without being recorded.

It is generally desirable to make the focusing stripes 143 wavy, insteadof straight, as shown. This avoids periodicity problems that could occurif the object being focussed upon is composed of alternate light anddark bands extending parallel to straight stripes 143, e.g., a picketfence. Also, if the present invention is incorporated into a single lensreflex camera, the photosensitive stripes 143 need only be present in asmall area at the center of the platen 36 which may be marked by aborder. When taking a picture, the camera is aimed so as to center thatborder on the point of most interest in the field of view. In this way,the focus and exposure settings will be determined by the distance andlighting conditions at that location.

In describing the operation of microscope-camera 10, we will assume thatthe operator has pressed the FORWARD button 50 to advance the tape 34while it is being repelled from platen 36 as discussed above untildetector 134 signals the presence of the first viewing frame at focalplane P. Upon receipt of that detector signal, control section 48 stopsdrive motors 42 and closes the tape gate (if present) thereby lockingthe first viewing frame 36b at the focal plane P.

The control section also initiates the focus routine described above bysampling the signals from the array of stripes 143 on platen 36 untilthe instrument is brought into exact focus on the desired object in thefield of view, i.e. specimen S. At this stage, the electron source 74 isin its retracted dotted line position in FIG. 6 so that the operator canexamine specimen S by looking through the eyepiece 68. The instrument isalso now ready to store a picture of specimen S on the first imagingframe 34a of the tape 34 if the operator wants to do this. In thatevent, he depresses the EXPOSE button 52 on the camera housing whichprompts the control section 48 to issue a series of command signals thatcontrol the various operative parts of the camera. More particularly,section 48 energizes and samples the signals from stripes 143 to developand store TF and ED signals as described above. From the TF signals,section 48 computes the adjustment for resistor 146 to bias the tape toestablish the requisite exposure field strength in the tape for theexposure duration called for by the ED signal. In other words itcustomizes the charging and duration to the prevailing lightingconditions and the range of densities of the object being viewed. Then,section 48 applies a drive signal to motor 78 causing the motor toextend the electron source 74 to its solid line position in FIG. 6wherein it overlies the focal plane P and blocks light entering themicroscope through eyepiece 68. Section 48 also applies drive signals toservomotors 42 to advance the tape, which advancement continues untilthe leading edge of the first imaging frame 34a is detected by detector136.

Control section 48 responds to the detection signal from detector 136 bydeenergizing motors 42 to stop the tape advance and by closing the tapegate (if present). That section also charges film layer 145 o plate 36so that the imaging frame 34a is now positioned at focal plane P andheld against the platen 36. That detector signal also prompts controlsection 48 to advance the frame counter 55 so that it shows the numeral"1". After section 48 receives acknowledgements indicating that all ofthe above operations have been completed, it energizes electron source74 with power from power supply 56, adjusts resistor 146 (FIG. 11) andcloses switch 122 for the duration of the ED signal thereby grounding byway of contact 118 and strip 112 the conductive layer 104a of the tapeframe at plane P. This applies at the beginning of the exposure no lessthan 5 volts across the frame to facilitate tunnelling of photogeneratedcharges through zone 106b. It also causes a cloud of electrons todescend toward, and uniformly charge, the exposed upper surface 106d ofthe film frame at plane P, while at the same time that frame receivesimaging photons through the lens unit 38. Resultantly, as describedabove in connection with FIGS. 11 and 12, a strong electric field isdeveloped in zone 106b so that positive carriers tunnel through thatzone and become pinned or trapped in zone 106a, approximately 100 Å intothat zone. Further, controlled by the value of the TF signal, source 74disperses a specific amount of negative charges during the exposureduration to equal the maximum number of photogenerated charges whichhave tunnelled through zone 106b, thereby establishing a chargeequilibrium in the storage zone 106a. Accordingly, a perfectly exposedelectronic equivalent image corresponding to the photonic imageprojected onto focal plane P is acquired by that tape frame and storedin its storage layer 106.

As described above, the electronic image is present on layer 106 as atopographical distribution of different-charge coulombic domains overthe area I of the tape frame 34a. This distribution is composed of twoparts, namely the charges which were deposited on layer 106 at thebeginning of the exposure step to establish the initial internal fieldbetween the surface 106d of layer 106 and electrode layer 104a, plus thephotogenerated charges created by exposure of the tape frame. Thus, thenumber of electrons at each point on the surface 106d equals the numberdeposited initially (circled in FIG. 11) plus a number of electronscorresponding to the number of photogenerated positive charge carriersthat tunnelled through zone 106b during the exposure step (uncircled inFIG. 11). In the normal mode of operation, the initial charge (circledin FIG. 11) remains on the tape frame 34a after the exposure step iscompleted, i.e., after electron source 74 is shut off and switch 122 sopened. Thus, the charges on zone 106a are spatially varied by thenumber of photogenerated carriers which became superimposed on theevenly distributed carriers present in thermal equilibrium initially.However, at each point on the frame 34a, the numbers of opposed positiveand negative charges are substantially equal.

After the exposure step, when source 74 is turned off and switch 122 isopen, thereby removing the negative bias that was set to controlelectron cloud current density and duration, the positive charges whichtunnelled through zone 106b are pinned in place in zone 106a, theretention time (t_(r)) being determined by the decay of the space chargelayer near the interface layer 106b, as follows:

    t.sub.r ≈ln 2/[v exp(qφ.sub.B /kT)]

where v is the dielectric relaxation frequency.

It should be noted that any free thermally generated or evenphotogenerated positive carriers now have insufficient energy (kT/q=26MeV) to tunnel through the zone 106b barrier (qφ_(B) =1.7V) and upsetthe stored charge count at the underside 106c of zone 106a. If there arestill any excess negative charges on the surface 106d of zone 106a,i.e., electrons with no opposed positive carriers at the underside ofzone 106a, these may be removed by means of a grounded conductive roller160' (FIG. 6) rotatively mounted in the bottom wall of cameracompartment 86 and touching the surface of zone 106a as the tape isadvanced automatically to its next frame position. It should be notedthat those electrons representing the image remain unaffected as theconductive roller passes over frame 34a.

Simultaneous with the recording of the picture on each tape frame asjust described, an electronic fiducial mark 128 is recorded in the top(i.e. right hand) edge margin of that frame outside the image area Ithereof as shown in FIG. 7. As will be described later, these marks 128,recorded at the same times as the images, enable the microscope-camera10 prior to each read-out operation, to set the initial position (zero)and skew of the scanning beam from electron gun 84 to compensate for anyslight mispositioning of each tape frame 34a at its position at plane Rwhen an image is read from the frame with respect to its position atplane P when that image was recorded on that frame. Microscope-camera 10records these marks 128 on the tape by means of a light unit 132 locatedin platen 36 at the righthand corner of compartment 64 at focal plane P.

As best seen in FIG. 7A, unit 132 comprises an elongated light source132a such as a LED or laser diode extending transverse to the tape 34and which preferably emits green (e.g. λ=500 nm) light. The othercomponent of unit 132 is an opaque mask 132b positioned to be inintimate contact with the tape in plane P. The mask has a precise narrow(e.g. 1 micrometer) elongated (e.g. 10 mm) slit leg 133a extendingtransverse to the tape (i.e. X axis) with a (Y axis) cross-slit 133badjacent the forward edge of platen 36. Each time an optical image isimpressed on the image area I of a tape frame 34a, control section 48energizes light source 132a so that the marginal area of tape frame 32aopposite slits 133a and 133b receives a saturating dose of light. Asshown in FIG. 14, at that applied voltage, the number of secondaryelectrons emitted from zone 106a exceeds the number of arriving primaryelectrons from source 74. Once the electrons are removed from thedarkest parts of the image areas (i.e., those circled electronsdeposited initially at the beginning of the exposure step), only theuncircled electrons remain which counterbalance the positive chargespinned to the underside of zone 106a. Thus, as shown in FIG. 15, onlythe charges corresponding to the image remain on the frame. In responseto incident light varying from 6·10 photons/cm² to 6·10⁹ photons/cm², atypical electronic image as in FIG. 15 might vary from 20electrons/pixel to 20,000 electrons/pixel, corresponding to a fieldstrength of 70 V/cm to 70·10³ V/cm inside the storage zone 106a. The netresult is that in the unexposed or dark portions of the frame, theoriginally applied 3·10¹¹ electron/cm² blanket charge is removed so thatthe stored image is completely free of this bias. The surface charge inthe exposed portions of the frame also drops to the exact same extent,but now reflects only the image information.

The magnitude of the dark current in modulator 104 during exposure andbias removal is temperature dependent and relatively small in comparisonto the charges created during exposure. A second picture may then bestored on the second imaging 34a of the tape with the frame counter 55being incremented to show a "2". In a similar manner, electronic imagescan be recorded in sequence on the remaining imaging frames 34a of thetape by repeatedly pressing EXPOSE button 52. After each such exposure,the next viewing frame 34b is moved to the focal plane P and the framecounter 55 will have been incremented by one. The tape 34 has typicallyseveral hundred or more sets of viewing and imaging frames so that alarge number of images can be stored on a single tape.

Also, if the operator wishes, he may skip frames if he chooses to do so.For this, he presses the FORWARD button 50 repeatedly causing controlsection 48 to actuate drive motors 42 to repeatedly step the tape toplace succeeding viewing frames 34b at plane P and to increment thecounter 55 until the counter displays the desired frame number. Theoperator can now view beforehand, and then take a picture of, a newspecimen which will be deposited on the next imaging frame 34a. Theskipped frames can then be returned to and used later by depressing theREVERSE button 51. This causes control section 48 to actuate the drivemotors 42 to step the tape backwards and to decrement counter 55 untilthe desired frame number is displayed by the counter, one may tapeforward or in reverse without exposing the tape by depressing button 50or 51. As each frame 34a moves past detector 136, the resultant detectorsignal causes control section 48 to increment or decrement the framecounter 55. When the selected frame number is displayed by the framecounter, the imaging frame 34a corresponding to that number ispositioned at the focal plane P. The operator may then depress the READbutton 53 which will cause control section 48 to advance the tape oneframe to place that selected frame at the read-out plane R in thedarkness of compartment 86. Then section 48 automatically executes aread-out routine. Further, it first energizes the electron gun 84 andits beam control circuit 88 in housing compartment 92 from power supply56 or from a remote power source via connector 62 (FIG. 5). Then, asbest seen in FIGS. 6-8, it closes a switch 157 which connects a contact158 in compartment 86 (and thus film layer 104a ) in a high voltage DCcircuit with gun 84 and power supply 56. In this circuit, the guncathode receives a voltage of about -2 KV, while collector 94 is atground potential and film layer 104b is held at a bias voltage of about300V. Resultantly, as shown in FIG. 6, electron gun 84, and moreparticularly its emission electrode 84a, located in an enclosure 84b,emits a small diameter (i.e., 2 micrometer) electron beam which impingesthe selected imaging frame 34a at read plane R. Cold cathode electronemission sources 84 which can be operated with very little power (about1 nanoampere) are known in the art.

As best seen in FIG. 6, on its way to the tape frame at read-out planeR, the focused electron beam e from electrode 84a passes between thevertical and horizontal deflection plates 84c and 84d of gun 84.Normally, a controlled voltage is applied to each set of plates by thebeam control circuit 88 so as to cause the electron beam e to sweep outa raster composed of parallel scan lines L (FIG. 7) on the imaging frame34a positioned at plane R, penetrating that frame's layer 106 to anexactly known depth. Where the beam impinges the frame, secondaryelectrons are emitted from layer 106a at that point. The electron beamoperates at the so-called second crossover point so that each primaryelectron results in the emission of one secondary electron from layer106. These secondary electrons form a return beam e' which is modulatedby the number of charges representing the electronic image stored onsurface 106d with its counter-charges at the underside 106c of thatframe 34a. In other words, the numbers of secondary electrons emitted ateach point on frame 34a impinged by the primary electron beam willdepend upon the number of charges and counter-charges stored at thatpoint on layer 106. More specifically, where the number of storedelectronic charges on layer 106 is larger, corresponding to a fiducialmark 128 or the lighter areas of the acquired optical image, there willbe fewer electrons needed in the primary beam to achieve the signallevel carried in the secondary emission e'. There is likewise anincrease in the number of primary electrons in the scanning beam from apoint on the swept frame area where there are fewer stored charges,corresponding to a darker area of the stored image.

The secondary electrons comprising the return beam e' strike collector94. Readout by secondary electron emission allows the employment in thecollector of an optimum performance, low noise amplifier such as adynode amplifier. This is a known electronic device consisting of asuccession of electron emitters arranged so that the secondary electronsproduced at one emitter are focused upon the next emitter. Thisamplifier thus produces a current output which is as much as one milliontimes stronger as the input represented by return beam e' and thus italso represents the amplified version of the mark 128 and the electronicimage stored on the tape frame 34a.

For each frame 34a, the amplified signal from collector 94 includes avery strong component corresponding to the fiducial mark 128 recorded onthe margin of that frame and a component corresponding to the electronicimage recorded in that frame's image area I. The former component isseparated out, say, by threshold detection, and routed to controlsection 48 where it is used to initialize the beam scan from gun 84 sothat the beam scan is always made with reference to the images on thetape rather than to the tape itself In this way, a slight mispositioningor skewing of the tape in its movement from plane P to plane R will notaffect the readout process.

More particularly, at the outset of each read-out operation, controlsection 48 causes beam control circuit 88 to execute a search routinewhereby that circuit moves the beam e in the X and Y directions over themargin of tape frame 34a until the collector 94 detects strong bursts ofsecondary electrons at the intersection of the crossarms 128a and 128bwhich constitutes the reference position of the beam scan. Circuit 88then causes the primary beam e to track along the X axis arm 128a of themark which is inherently parallel to the filter stripes 142 throughwhich the image on that frame was exposed. This ensures that when thebeam e sweeps over the image area I during read-out, the beam scan lineswill be parallel to those frame exposure lines. The circuit 88 thenstarts the beam scan at the corner of image area I closest to the mark128 which is offset a constant distance from the aforesaid zeroposition, i.e., the "electronic cross-hairs" 128a and 128b.

During the scan of image area I, the picture signal component fromcollector 94 is applied to an A/D converter included in read-out circuit96 in housing compartment 98 and is otherwise processed by circuit 96 toprovide a picture signal. When a color image is being read from a frame34a, control circuit 88 controls the electron gun 84 so that theelectron beam e scans the electronic image on frame 34a in threesuccessive operations. First the beam scans the frame where it wasexposed through all of the red filter lines (R); then it scans the framelines that were exposed through the green filter lines (G), and finallyit scans the portions of the frame area that were exposed through theblue filter lines (B). The three successive scans produce a set of red,green and blue picture signals corresponding to the image on that frame.These signals are digitized and, after being color corrected in circuit96, they may be applied to terminal 20 (FIG. 5) to print or displaycolor pictures corresponding to the images stored on tape 34.Alternatively, if separate long-term storage of the picture signals readfrom the tape frame is required, the signals may be applied viaconnector 21a to a conventional video disc or video tape drive.

The initial zeroing of the electron beam e that scans the tape frame tobe read at plane R using the electron fiducial mark 128 recorded alongwith that image assures that the scanning electron beam e will sweepacross the tape frame in register with the lines on that frame that wereexposed through the color filter stripes 142 when the tape frame was atplane P. If desired, however, additional beam control may be obtained byrecording tiny fiducial marks 160 (FIG. 9) on a non-imaged side marginof the tape frame which are congruent with each red, green and bluefilter stripe 142 when the frame 34a is positioned at focal plane P. Inthis event, the read-out circuit 96 would include a discriminator toseparate the color picture signals read from the image area I of filmframe 34a and the scan line position signals read from that frameoutside the area I. The latter signals are then processed by electrongun control circuit 88 to control, in a correctional feedbackarrangement, the deflection voltages applied to the electron gun'sdeflection plates 84c and 84d to correct for any misregistration of thescanning beam e with the frame lines corresponding to the color filterstripes 142.

The detection threshold of collector 94, i.e. its sensitivity, is suchthat each individual secondary electron can be detected and amplified sothat the amplification factor of the resultant signal from collector 94can be as high as 10⁶ or more. Thus, the read-out process carried out byinstrument 10 involving detection of secondary electrons emitted fromtape 34 is totally different from the prior scanning methods describedat the outset which detect a capacitively modulated current signal froma recording medium. By detecting and simply counting individualelectrons in a return beam instituted by the charge distribution on tapesurface 106d of frame 34a, rather than current flow through the frame,the present apparatus can take advantage of the highly sensitivedefect-free nature of the tape frame 34a, to produce a picture signalwhich has extremely high resolution and information content.Furthermore, it can accomplish this at a lower read-out or scanningvoltage, thereby conserving battery power.

In some applications, the scan control circuit 88 can be arranged tocontrol the beam from gun 84 so that it scans two different rasters. Arough scan, say, every other or every third color line, may be executedfor each color to provide picture signals suitable for previewing onterminal 20 to see if the correct image is being read-out. Then, if theimage is correct, a regular scan at the finer resolution may beperformed to reproduce a hard copy of that image.

In a preferred embodiment of my system, means are provided forincreasing the beam current in the beam e from gun 84 while that beamdwells at each picture element or pixel in its scan across frame 34a soas to extend the dynamic range of the system's charge detectioncapabilities. This is desirable if more charges per pixel are present onthe tape frame than can be handled by the usual lower beam current. Moreparticularly, the read-out circuit includes a threshold detector whichcounts the number of secondary electrons emitted from each pixel over atime period equivalent to a fraction, e.g. one-half, of the dwell timeof the beam at that pixel. If the threshold is exceeded, the detectorissues a signal to control section 48 causing that section to double thecurrent in the beam from gun 84 for the remainder of the dwell time atthat pixel. Such doubling will thereupon increase the dynamic range ofthe system by a factor of 10 to ensure that it will not be saturated oroverloaded by especially strong image signals on the tape.

Unlike prior systems, when instrument 10 scans a frame 34a duringread-out, it does not destroy the electronic image stored on that frame.On the contrary, it automatically refreshes that image which can thus beread over and over again. This is because during scanning, which takesplace in the darkness of compartment 86, there are no photo-inducedelectron-hole pairs produced in the medium's modulator 104; nor is thereany buildup of charge on the medium's layer 106 since the beam operates,by choice, at the second crossover point as mentioned above.Resultantly, the positions of the positive charge carriers (holes) atthe underside 106c of storage zone 106a remains undisturbed, while thenegative charges at the surface 106d of that layer are continuallyreplenished by electrons in the electron beam to maintain a chargebalance across the layer 106 at each point thereon as depicted in FIG.15. As a consequence, the field strengths of the charge domainsdistributed on layer 106 of each frame 34a are maintained, allowingtheoretically infinitely repeated read-outs of that frame.

Indeed, the electronic images stored on unread frames 34a can berefreshed or renewed from time to time by repositioning each such frameat focal plane P and flooding it again with electrons from electronsource 74 with the switch 122 (FIG. 8) remaining open so that thatframe's conductive layer 104a is not grounded. Those beam electrons willreplace any electrons on the outer surface 106d of storage layer zone106a that may have leaked away over time so that the negative chargedistribution on that surface will again correspond to the distributionof positive carriers still present at the undersurface 106c of thatzone.

Instead of retrieving the image stored on the tape 34a by electron beamscanning as shown, the tape can also be read by detecting so-called"tunnel electrons" using a sensing needle that is caused to scan acrossthe surface 106d of tape layer 106. As the needle moves across thatsurface, an electron cloud is present in the gap between that surfaceand the needle tip as a consequence of the stored electrons' wave-likeproperties. Resultantly, there is a voltage-induced flow of electronsthrough the cloud which varies from point to point on the tape,depending on the charge stored thereat. This electron tunnelling anddetection phenomenon is described in greater detail in ScientificAmerican, August 1985, pp. 50-56. Using this technique, electrons can be"picked off" the frame surface 106d at each point on the frame toproduce picture signals corresponding to the image recorded on theframe.

Microscope-camera 10 with its recording medium can be used in a varietyof ways. It can be used for long or short term data storage, asdescribed above. It can also be used for buffer storage or to effectcomparisons between the same optical image recorded at different times.For example, a picture of specimen S recorded on one tape frame 34a canbe read-out to one channel of a terminal 20 with a two channelcapability. Then, the same specimen can be recorded at a later time onanother tape frame 34a and immediately read-out to the other channel ofterminal 20 so that the two pictures of specimen S can be displayed sideby side. The output signals, also produced by instrument 10 during aread-out operation, can be processed digitally using means well known inthe color graphics industry to produce an enlargement of the storedimage or any selected area thereof or to generate pseudocolor and falsecolor variations of the stored image. In addition, as alluded to above,the present invention can be incorporated into a single lens reflexcamera. In this event, the electron gun 84 would be located in the samecompartment as the instrument's primary lenses. In other words, thefocal plane P and the read-out plane R would be the same. The camera'sviewing optics, on the other hand, would be located in a compartmentbranching from the main compartment 64 with appropriate mirrors andlenses to permit the operator to look through the camera eyepiece to theback of a film frame 34b positioned at the camera's focal plane. Also,an appropriate shutter would be provided to isolate that branchcompartment while the aforesaid exposing and read-out processes arecarried out in the camera. Also, in such a camera, the filter stripes142 (R, G, B) can be applied to the exposed surface of the filmsubstrate 102 rather than to platen 36, as described above, to simplifyregistration of the scanning beam with the filter lines during read-out.

It is important to understand that the reading out of the electronicimage stored on the medium 34a by the detection of secondary electronsis quite unlike the scanning processes used in the prior systemsdiscussed at the outset which develop a capacitively modulated currentsignal. This readout technique disclosed here is made possible only withthe development of the unique recording medium 34a described herein.This is because only this medium achieves the crystalline perfection inits various layers that enables the storage of the electronic image asprecisely distributed minute charge domains on layer 106 as describedabove. Using a finely focussed scanning electron beam, these minutecharges produce the requisite high energy but low voltage modulation ofthe numbers of secondary electrons emitted from successive points in thebeam scan to produce a picture signal with an information contentcomparable to that in the stored image. Furthermore, this can beaccomplished using a relatively low voltage battery supply. Therefore,the medium of this invention is particularly suitable for incorporationinto portable recording apparatus such as a microscope or camera.

Thus, medium 34a meets completely the objectives set forth at theoutset, namely, it obtains the benefits of both direct and indirectrecording. More particularly, it acquires and stores the incoming lightimage in an analog form that retains all of the information in theincoming light wavefront and achieves maximum conversion of thatincoming light energy to the electronic image-representing chargepattern stored in the medium. Yet, the medium permits retrieval of thatstored information as a serial electrical signal that can be stored orhandled with the same ease as the signals in present day audio and videoread/write systems.

In my medium, the silicon modulator 104 is typically grown to athickness slightly in excess of one micron, thus allowing for thepanchromatic or total absorption of incoming light over the entirewavelength range from ultraviolet to infrared. Also, the photoconductiveproperties of the silicon-based modulator allow for or provide anefficient response to incident light in this range. Indeed, fewer thansix photons incident on the medium suffice to initiate a photoconductivereaction in modulator 104 and the quantum conversion efficiency of themedium approaches 100% as noted above. Furthermore, its purity,perfection and freedom from electrical noise combine to give the mediuman exceptionally high signal-to-noise ratio, yielding a grey scale ofover three decades in three colors. In fact, the medium is sensitiveenough to acquire and store optical images in black and white or incolor even under moonlight conditions and its response is comparable tothat of photographic film having an ASA rating of 1000 or more.Furthermore, my medium suffers minimal hysteresis loss and fatigue whenthe image is erased from the medium by U.V. light as aforesaid.Therefore, it can be reused repeatedly without any appreciable loss ofsensitivity or responsiveness.

Because of its many attributes described above, the medium disclosedherein should find wide application not only in image recording systems,including microscopes and cameras, but also in a variety of other areas,such as fiberoptic signal receiving systems, large bandwidth digitalrecording systems and optoelectronic switches.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained. Also,certain changes may be made in the above construction and in the methodset forth without departing from the scope of the invention. Forexample, gallium arsenide may be used as the photoconductive material inmodulator 104 relying on the "direct band gap" absorption properties ofthat material. Or, if sensitivity of the medium 34a to long waveradiation is of importance, the light modulating layer 104 may becomposed of cadmium mercury telluride.

It should be appreciated from the foregoing also that my invention canbe implemented as a recording medium which records and stores directlyincoming electrical signals. For this application, first a conductivelayer 104a and then a dielectric storage layer 106 are deposited on base102. Electrical signals may be recorded on the medium by "writing" onthe surface of zone 106a using a point source of electrons connected ina D.C. circuit with the conductive layer 104a. This produces a chargedistribution on the storage zone similar to the charge pattern on layer106a in FIG. 14 described above. Therefore, it is intended that allmatter contained in the above description or shown in the accompanyingdrawings, shall be interpreted as illustrative and not in a limitingsense.

It will also be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. An optoelectronic recording medium in tape orstrip form of the type including a base and one or more layers added tothe base, at least one of which is photoconductive, the improvementwherein the base consists solely of a thin, web-like monocrystallinesapphire crystal whose c-axis is oriented perpendicular to the nominalplane of the medium and whose thinness is such that it is flexible, yetof sufficient strength to support said added layers so that the mediumcan be formed into a roll having a relatively small diameter.
 2. Themedium defined in claim 1 wherein said base is transparent to lightenergy over a broad wavelength spectrum.
 3. The medium defined in claim2 wherein said medium has at least one other layer which is transparentto light energy.
 4. The medium defined in claim 2 wherein said addedlayers include at least one conductive layer, a photoconductive layerand a dielectric layer, in that order.
 5. An optoelectronic recordingmedium comprising a strong flexible sheet or tape structure formableinto a roll composed of a plurality of thin, superimposed,hetero-epitaxially grown regions or zones having different electricalcharacteristics.
 6. The medium defined in claim 5 wherein said structureincludes an electrically insulating substrate region, at least oneconductive region, and a photoconductive region, in that order.
 7. Themedium defined in claim 6 and further including a dual-materialdielectric region added to and superimposed on said photoconductiveregion.
 8. The medium defined in claim 7 wherein said dielectric regionincludes a barrier zone adjacent to said photoconductive region forinhibiting movements of charge carriers.
 9. The medium defined in claim7 wherein said substrate region is transparent and optically clear tolight energy over a spectral range from ultraviolet to infrared.
 10. Themedium defined in claim 7 wherein said recording medium alternates witha transparent medium as a continuous tape or sheet so as to permitviewing through said transparent medium before recording on saidrecording medium.
 11. The medium defined in claim 10 wherein eachtransparent medium on the structure consists of said substrate regiondevoid of said other regions.
 12. The medium defined in claim 7 whereinthe dielectric region is comprised of a material which is renderedelectrically conductive by exposure to ultraviolet light.
 13. The mediumdefined in claim 7 wherein said dielectric region has a sufficientlyhigh degree of perfection as to accept and maintain extraordinarilyuniform electronic charges on its surface with minimal lateral migrationof said charges.
 14. The medium defined in claim 13 wherein saiddielectric region is composed of a material selected from the groupconsisting of silicon nitride, silicon dioxide and sapphire.
 15. Themedium defined in claim 6 and further including electrical conductormeans at an exposed edge of said medium, said conductor means beingconnected electrically to said conductive region.
 16. The medium definedin claim 6 wherein said substrate region is a web-like monocrystallinesapphire crystal having its c-axis orthogonal to the nominal plane ofsaid medium.
 17. The medium defined in claim 6 wherein saidphotoconductive region is composed of a material selected from the groupconsisting of silicon, gallium arsenide and cadmium mercury telluride.18. The medium defined in claim 6 and further including optical filtermeans on said medium, said filter means comprising a multiplicity ofparallel, contiguous, thin, optical filter lines, said linesA. beingcoextensive with said medium; B. being responsive to different colors;C. extending from one edge of the medium to the opposite edge thereof;and D. being arranged in a repeating color sequence on said medium. 19.The medium defined in claim 6 and further including optical and/orelectronic marks on said medium for controlling the frame position ofthe medium in the camera system and/or the read-out of image on themedium.
 20. The medium defined in claim 19 wherein at least some of saidmarks are coded to designate frame numbers on the medium.
 21. A plurallayer flexible optoelectronic recording medium in strip or tape formcomprisingA. a thin base layer consisting of a single flexible web-likemonocrystalline sapphire crystal whose surfaces are substantially defectfree; B. an organic conductive layer grown as a crystal on a surface ofthe base layer from nucleation sites on said surface so that theconductive layer has a crystal arrangement that is compatible with thatof said base layer and has a high degree of perfection; C. aphotoconductive layer added as a continuum to the exposed surface ofsaid conductive layer and being capable of electrically modulating anincident light image; and D. a dual-material storage layer addedintegrally to said photoconductive layer for capturing electronic chargecarriers from said photoconductive layer and thereby storing anelectrical analog of said incident light image.
 22. The medium definedin claim 21 wherein said base layer and said conductive layer aretransparent to light energy and the photoconductive layer absorbsincident light energy.
 23. The medium defined in claim 21 wherein all ofsaid layers in the medium have compatible atomic lattice spacingsforming a coherent hetero-epitaxially grown structure.